In this project we examine how immunological memory formed during one infection is protected from erasure in the face of repeated subsequent infections by multiple unrelated pathogens. This is significant because stable memory is essential for immunizations to successfully confer lifelong protective immunity. Understanding this mechanism will allow us to improve vaccines that otherwise require frequent boosting and even develop new ones for diseases such as malaria. Although cytokines such as IL7 and IL-15 maintain the absolute size of the memory T cell pool in the immune system, the pathways ensuring that the population includes adequate representation of all the previous experiences, is not known. Based on our recent work, we hypothesize that the maintenance of a diverse CD4+ memory repertoire requires trophic interactions of memory T cells with endogenous sub- threshold ligands (STLs). STLs are distinct peptide-MHC complexes that are too weak to stimulate conventional T cell activation. Since only a small proportion of memory cells engage a particular STL, competition for survival is limited to these small colonies of T cells. The significance of this mechanism is that it avoids a much broader repertoire loss that might happen if the competition was more widespread. We have generated unique reagents and developed methodologies to test this innovative hypothesis using two major independent and complimentary aims. 1. Establish how TCR-specificity determines which T cells can compete during memory maintenance, using recently identified TCRs that either share antigen-specificity or STL-specificity. We will examine the cellular mechanisms for such competition and establish the range of impact it has on a polyclonal repertoire of T cells. 2. Using sequential infections with Plasmodium, Influenza and Toxoplasma, evaluate how pre-existing memory T cells can be destabilized by the response to a new infection. We will also explore how STL- treatment can help restore the stability of memory T cells in this context. On completion, we expect to offer a novel understanding of CD4+ memory maintenance and suggest strategies to improve memory T cell survival during adoptive therapies and vaccinations. The converse strategies can help decrease the frequency of pathogenic T cells during autoimmunity and transplantation.